Chemical mechanical polishing is one of the many steps commonly used in the manufacture of integrated circuits. As detailed in many prior art patents, chemical mechanical polishing, or simply "CMP," is the process of polishing the surface of a semiconductor wafer in order to remove material from the surface of the wafer. The polishing is typically performed by rotating a polishing pad against the semiconductor wafer. A slurry of some sort is used to facilitate the polishing process. Depending upon the material that is to be removed from the semiconductor wafer, the composition of the polishing pad and the composition of the slurry varies.
For example, in the CMP of tungsten material from the surface of a wafer, the slurry will include an oxidizer, which is typically ferric nitrate crystals (Fe(NO.sub.3).sub.3). The ferric nitrate crystals are usually diluted in deionized water and then mixed with aluminum oxide (Al.sub.2 O.sub.3) before being introduced into the CMP apparatus. Examples of slurry compositions are detailed in U.S. Pat. No. 5,783,489 to Kaufman et al. and the patents cited therein.
Typically, before the slurry enters the CMP apparatus, a physical filter is used to remove any large particles that may scratch the surface of the wafer. Nevertheless, even with this filtering, defects caused by particles are found on the surface of the semiconductor wafer. These defects can result in short circuiting of metal interconnect layers. The defects may be singular or may be of the "skipping stone" type.
It is theorized that the root cause of this defect mechanism is the formation of iron oxide in the slurry. It is believed that the iron oxide is generated from the raw ferric nitrate material itself. In other words, due to the nature of ferric nitrate manufacturing, iron oxide is easily formed during the precipitation process. These impurities are specified as "insoluble material" in the product content table. Some of these particles may escape the physical filter and reach the wafer surface.
It is also believed that the ferric nitrate may be decomposed and oxidized into iron oxide and accumulate on piping that leads from the slurry reservoir to the CMP apparatus. After some amount of sedimentation and agglomeration, the iron oxide releases from the piping wall and reaches the CMP apparatus and the wafer surface. The decomposition may result from a change in pH level, heat, light, and other particle interaction.
As noted above, the iron oxide particles may be partially removed by physical filtration. However, this method can be prohibitively costly as the fineness of the physical filter decreases. The iron oxide deposited on the piping wall downstream from the physical filtering cannot be removed by the filters and will require either frequent pipe changing or chemical cleaning.
Therefore, what is needed is a method of removing the iron oxide particles from the slurry.